As network connection devices, there are devices using a wired IP (Internet Protocol) network and devices using a wireless IP network. Power-saving techniques for reducing power consumption are under study for various devices, irrespective of above-described wired or wireless devices. Since wireless portable type terminals among such devices are expected to be used in such a manner that they are operated only by a built-in battery for many hours, a power-saving technique is particularly necessary. Therefore, examples of power-saving techniques for portable terminals will be described below.
As network connection devices of portable terminals, attention is being focused on the use of wireless LAN (Local Area Network) devices. Wireless LAN devices consume relatively large power. Therefore, power saving is an important issue to be studied for wireless LAN devices and many power-saving techniques are being proposed.
In IEEE802.11, which is a wireless LAN standard specification, a power management mode (power-saving mode) is defined as a power-saving method for a wireless LAN terminal provided with a wireless LAN device. According to this method, a wireless LAN terminal operating in a power-saving mode transitions to a normal transmission/reception operation state (active mode) according to timing at which a beacon is transmitted from a wireless base station, which is an access point, and performs reception operation. The wireless LAN terminal then enters a state in which it does not perform reception operation for a period during which no beacon is transmitted from the wireless base station (hereinafter, referred to as “power-saving mode”). In the power-saving mode (also referred to as “doze mode”), the wireless LAN terminal stops power supply to a wireless LAN device provided for the wireless LAN terminal, and can thereby reduce power consumption of the wireless LAN terminal.
When a wireless LAN terminal operating in a power-saving mode is connected to such a wireless LAN system, an access point increases a period during which a beacon is transmitted (beacon period) and the number of intermissions of beacons until a DTIM beacon is transmitted. Here, the DTIM (Delivery Traffic Indication Message) beacon is a message that transmits to the wireless LAN terminal in the power-saving mode the fact that data is queued for transmission. This allows a power saving effect to be expected from the wireless LAN terminal.
However, when a long beacon period is set, a large delay time is generated in the reception of unicast data.
For example, Wi-Fi WMM-APSD (WMM Power Save (see NPL 1)) and a U-APSD (Unscheduled Automatic Power-Save Delivery) scheme in the IEEE802.11e standard (IEEE Std 802.11e-2005 (see NPL 2)) are known as methods to solve this problem. These two standards define a method of realizing power saving for unicast data. The method uses a method for transitioning to an active mode independently of a beacon period in consideration of priority control for each application, and delay and range of fluctuation to maintain communication quality of the application. This allows the wireless LAN terminal to realize power saving for unicast data.
The wireless LAN terminal using this method transitions from a doze mode to an active mode to maintain communication quality for each application at its own transmission/reception period. The wireless LAN terminal that has transitioned to the active mode requests the access point to transmit unicast data directed to the wireless LAN terminal.
To be more specific, the Wi-Fi WMM-APSD switches to the active mode at an interval determined between the access point and the wireless LAN terminal to transmit/receive data. As the method of dynamically determining the interval at which switching is made to the active mode (e.g., PTL 1), a method is proposed whereby a wireless LAN terminal analyzes communication data resulting from communication with an access point. The wireless LAN terminal detects communication with the access point or a state of a communication protocol. The wireless LAN terminal determines the interval at which communication data is received from the access point by referencing a reception interval table prepared beforehand for each state according to the communication or the state of the communication protocol.
Furthermore, the U-APSD receives data triggered by a trigger frame transmitted from a wireless LAN terminal in a power-saving mode. The trigger frame is created as follows.
FIG. 1 is a diagram illustrating a data format of transmission data used in a network layer of a wireless LAN terminal. As shown in FIG. 1, the transmission data includes an IP header and IP data. Furthermore, the IP data includes a TCP (Transmission Control Protocol) header and TCP data. Here, when the transmission data is an acknowledgment response, the TCP data is empty. On the other hand, when the transmission data is application data, the TCP data is data such as streaming data. It is possible to identify whether the transmission data is an acknowledgment response or application data by monitoring the data size of the TCP data.
FIG. 2 is a diagram illustrating details of the TCP header. In FIG. 2, a “sequence number” field is a field indicating a sequence number of the transmission data. An “acknowledgment response number” field is a field used when the transmission data is an acknowledgment response. The “acknowledgment response number” field stores a sequence number stored in the “sequence number” field in the TCP header of received data targeted for an acknowledgment response. Furthermore, a “flag” field is a field that stores SYN (Synchronize) or FIN (Fines) data or the like. Here, the SYN data indicates a request for starting TCP communication and the FIN data indicates a request for ending TCP communication. Furthermore, a “window size” field is used to transmit a window size on the receiving side to the other party. When the window size is 0, this indicates that data cannot be received. Thus, by monitoring the TCP header, it is possible to confirm whether the transmission data is an acknowledgment response or application data.
Furthermore, FIG. 3 is a diagram illustrating details of the IP header. In FIG. 3, a “service type” field is a filed used to specify TOS (Type Of Service) indicating the priority or the like of an ITP packet. The U-APSD classifies packets from a higher layer into access categories and stores the packets in their respective queues. There are four access categories; “speech,” “video,” “best effort” and “background,” and the access categories used are determined by an application. In the case of, for example, streaming communication, “video” is set as the access category. The access categories are associated with their respective priorities. For this reason, the access categories are used in priority control of a wireless LAN. The wireless LAN terminal using the U-APSD adds information indicating the access category to the “service type” field of the IP header of transmission data as synchronization information and transfers the field to the wireless LAN device.
Upon receiving the transmission data with synchronization information added to the “service type” field of the IP header of the transmission data, the wireless LAN device rewrites the wireless header as a U-APSD-compatible wireless frame. FIG. 4 is a diagram illustrating a data format used in a wireless LAN device using the U-APSD, that is, data link layer. Thus, when synchronization information is added to the IP header, the wireless LAN terminal generates a trigger frame by rewriting the wireless header to a U-APSD-compatible header. The frame generated is then recognized as a U-APSD-compatible trigger frame in communication with the access point.
The wireless LAN terminal using the U-APSD is usually in a doze mode and is switched to an active mode only when there is data to be transmitted. When, for example, the wireless LAN terminal is in the doze mode, even if there is U-APSD-compatible data addressed to the wireless LAN terminal at the access point, the access point cannot transmit the data to the wireless LAN terminal. Therefore, the data remains stored in a buffer of the access point.
When data addressed to the access point is generated in the wireless LAN terminal, the wireless LAN terminal is switched to the active mode and transmits data to the access point as U-APSD-compatible data. At this time, the access point knows that the wireless LAN terminal has entered the active mode and transmits the data stored in the buffer to the wireless LAN terminal. After that, the wireless LAN terminal transitions to the doze mode again.
Thus, the wireless LAN terminal continues a reception operation in an awake (transitioned to the active mode) state for data transmission, and can thereby perform transmission/reception simultaneously through one rising. When the data that has arrived at the access point is not U-APSD-compatible data, the wireless LAN terminal performs a normal reception operation in conformity with the operation in the power-saving mode or the like.